Very Low Intermediate Frequency (VLIF) receivers are popular due to their relatively low cost and small size, as well as their ability to operate over a broad range of frequencies. A VLIF receiver combines a received radio frequency (RF) signal with a local oscillator prior to analogue to digital conversion. The local oscillator is set at a frequency close to that of the RF signal, but differing by an offset that is referred to as an intermediate frequency. For example, the intermediate frequency is typically between 3.6 kHz to 5.7 kHz, depending on-channel spacing.
The local oscillator thus operates at a frequency either just above or just below the frequency of the RF signal, i.e. on one “side” of the RF signal. At times, this results in interference from an adjacent channel interferer on the side of the RF signal where the local oscillator is operating.
The RF signal comprises an in-phase component, I, and a quadrature component, Q. Often during down-conversion, noise, distortion or imbalance is introduced into, or pronounced in, the I and Q components. I/Q imbalance algorithms exist that attempt to estimate amplitude and phase imbalances in the RF signal and compensate for the imbalances. However such algorithms work adequately only under certain conditions.
The local oscillator can be changed to the other side of the RF signal, to avoid interference with the adjacent channel interferer. However, switching the local oscillator during a call, for example, can cause bad user experiences or result in a dropped call. Furthermore, another adjacent channel interferer can be present on the other side of the RF signal, potentially resulting in more interference after changing the local oscillator.
A further problem with VLIF receivers of the prior art is that detecting interference and changing the local oscillator frequency based upon the detected interference is time consuming, which can in turn result in a negative user experience.
Yet a further problem with VLIF receivers of the prior art, is that they are prone to falsely detecting a carrier. This can cause the VLIF receiver to incorrectly change channel, and thus in turn cause a gap in received data.
Accordingly, there is a need for an improved VLIF receiver and a method of controlling a VLIF receiver.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.